Generally, a converter for high voltage uses a power semiconductor, which is turn-on/turn-off controlled for mutual conversion between AC voltage and DC voltage. Because the withstand voltage of the power semiconductor is limited, multiple semiconductor modules having a power semiconductor circuit should be connected in series to process the high voltage. Various semiconductor modules may be connected with each other for a power semiconductor circuit configuration.
As it is known, in the case of a well-known modular multilevel converter (MMC), the power semiconductor circuit includes multiple sub-modules that form two output terminals, and the multiple sub-modules are connected to each other in series. Such sub-modules may be configured to include, for example, an energy storage unit and a power semiconductor circuit, which is connected to the energy storage unit in parallel and comprises multiple power semiconductor switches and free-wheel diodes.
When a certain sub-module fails among these multiple sub-modules, the failing sub-module is shorted to stably operate a system. For a short circuit, a phase current bypasses the failing sub-module by a bypass switch, and the system continuously performs normal operations by other normal sub-modules.
In this case, a technique for quickly bypassing a failing sub-module is very important in terms of a system as well as the corresponding sub-module. If the switching time of the bypass switch is slow, overvoltage and overcurrent are generated in the failing module. Accordingly, the internal components (for example, capacitors, power semiconductors, etc.) of the corresponding sub-module may explode and may lead to a fire. Also, as overvoltage and overcurrent are generated in the whole system, and overvoltage and overcurrent respectively exceeding a rated voltage and a rated current are applied to each sub-module, the reliability of the system is adversely affected. Therefore, the bypass switch must quickly operate and must be stable, but the conventional art may not maintain desired performance due to technological limitations.
Recently, various circuit configurations are proposed for the bypass of a phase current when a sub-module fails. However, these have unilateral circuits and it is difficult to perform an effective bypass operation.